The present invention relates to a control method and device for an electrically controlled automatic transmission used in an automobile, wherein the gear changing operation of the automatic transmission is executed by the change-over action of change-over valves driven by pilot-type solenoid valves actuated by magnetization or demagnetization of the solenoids, according to commands from a control circuit.
In the prior art of control devices of this type, small magnetic valves of the poppet-type were used as the pilot valves because, if the shift valve which works to select a desirable gear ratio was directly driven by the solenoid, a large electric current had to be applied through the solenoid which resulted in undesirable heat generated thereby.
As the solenoid valve of this type fulfills its pilot function by returning a part of the pressurized oil back to a tank according to magnetization or demagnetization of the solenoid, there is a possibility that the desirable oil pressure required in a total control system of the automatic transmission can not be maintained, if a plurality of magnetic pilot valves simultaneously open to return the pressurized oil back to the tank.
The maximum oil pressure required in a control system for an automatic transmission used in an automobile occurs at stall starting, however, the engine, at the moment of starting, does not rotate at a rotation speed higher than the stall rotation speed determined by a torque-convertor capacity. Therefor, in order for the quantity of oil fed by a pump to compensate the total oil quantity returned back to a tank in the total control system as well as, at the same instance, to maintain the required maximum oil pressure, one should either design the pump to have sufficient capacity or design the control system in a manner such that the returning oil quantity is minimized. From the standpoint of saving energy, the latter is better than the former.
On the other hand, considering the possibility of accidents or trouble like incomplete contact or disconnection of an electric line connected to the solenoid of the magnet valve, it is desirable to design the system to have a gear ratio for high speed, as high as possible, at demagnetization of the solenoid. This is easily understood from the fact that, in the system so designed to have a gear ratio for low speed at demagnetization of the solenoid, the automatic transmission, if there is a malconnection or disconnection of an electric wire connected to the solenoid during high speed driving, shifts suddenly down to the gear ratio of low speed which would cause damage to the engine by overrunning.
Moreover, in order to design the magnet valve, used as a pilot valve, to be small and simple, it is preferable to design it such that an armature is pulled into the solenoid in response to magnetization thereof to open the poppet.
To design the control system complying with the abovementioned conditions, in the case of a three-phase gear-ratio automatic transmission, the control system must have a function such that all the solenoids are demagnetized at the third speed (top gear) allowing, theoretically, no quantity of oil to return from the pilot valves and, on the other hand, they are all magnetized at the first speed (low gear) letting the maximum quantity of oil to return from the pilot valves.
In this design, however, the quantity of returning oil becomes maximum at stall starting so that it is quite difficult to get sufficient oil pressure required for the total control of the system at stall starting as mentioned above.